In the prior art, printed circuit board laminates have been utilized as a convenient and low cost means for mounting and interconnecting discrete electrical components. More specifically, printed circuit boards, formed of a dielectric substrate, are provided with conductive metallic pathways which define electrical connections between discrete components mounted thereon. The metal leads of the components may be soldered to the conductive pathways to complete the electrical connections.
The dielectric substrate used to form the printed circuit board is generally glass fiber cloth which has been impregnated with a resin formulation, such as epoxy or polyimide. The thermal coefficient of expansion of the dielectric substrate is significantly greater than that of the discrete components. The dissimilarity between the thermal expansion coefficients has not presented an insurmountable problem heretofore, since the flexibility of the metal leads of the discrete components would compensate for the thermal mismatch. Even in situations where the circuit assemblies are subjected to frequent and great thermal changes or excursions, a manufacturer can compensate for the thermal mismatch by providing a circuit layout design which incorporates expansion loops in the component leads to absorb the varied expansions and contractions of the elements thereby preventing stress on the solder joints which is a major cause of circuit failure.
Recently, manufacturers have developed leadless components such as chip resistors or chip capacitors. When the leadless chip components, generally formed from alumina, are directly affixed to a circuit board or to a conductive layer in a multi-layer laminate assembly, it has been found that the difference in thermal coefficients of expansion between the dielectric substrate and the alumina leadless chip components, has resulted in a high degree of circuit failures. More specifically, the thermal coefficient of expansion for a conventional epoxy glass laminate dielectric substrate is in the range of 15-20.times.10.sup.-6 inch per inch per degree Celsius. In contrast, the alumina chip components have a much lower thermal coefficient of expansion, generally about 6.times.10.sup.-6 inch per inch per degree Celsius. Thus, when a circuit board laminate having leadless components is subjected to high thermal excursions, solder joints between the components and the laminate frequently failed since there was no flexibility between the components and the laminate to compensate for the varying amounts of expansion. Therefore, it is apparent that it would be desirable to provide a circuit board laminate having thermal coefficient of expansion which more closely matches the thermal coefficient of expansion of the alumina chip components to prevent failures of the electrical connections. Further, it would be desirable to provide a circuit board laminate which continues to utilize glass epoxy substrates for the circuit board construction since the latter offers considerable cost advantages and are relatively easy to manufacture.
Accordingly, it is an object of the subject invention to provide a new and improved multi-layer circuit board laminate having a lowered thermal coefficient of expansion which is particularly useful in conjunction with leadless components.
It is a further object of the subject invention to provide a multi-layer circuit board laminate which includes one or more metal core layers for reducing the thermal coefficient of expansion of the laminate.
It is another object of the subject invention to provide a multi-layer circuit board laminate which utilizes a metal core to reduce the thermal coefficient of expansion and which includes a conventional glass epoxy printed circuit board, thereby retaining the advantages of the latter.
It is still a further object of the subject invention to provide a new and improved method for making a multi-layer circuit board laminate which includes a metal layer that reduces the thermal coefficient expansion of the laminate.
It is still another object of the subject invention to provide a multi-layer circuit board laminate which includes a stabilizing layer formed of a composite, metal-dielectric laminate enabling the layer to be provided with a non-contiguous floating-type pattern.